The invention relates to a switching arrangement for igniting and operating a high-pressure discharge lamp, the switching arrangement comprising:
input terminals for connecting a power supply source,
output terminals for connecting the lamp to be operated,
a flyback converter comprising a semiconductor switching element and a transformer, a primary winding of which is electrically connected to one of the input terminals and a secondary winding having a secondary winding voltage is electrically connected to one of the output terminals,
a control circuit for generating a switching signal for controlling the semiconductor switching element in a conducting or a non-conducting switching state.
A switching arrangement of the type described in the opening paragraph is known from EP 0 746 186. The known switching arrangement is suitable for igniting and operating a car headlamp. The lamp may be directly connected to the output terminals. However, the lamp is usually incorporated in a commutation network in the form of, for example, a bridge circuit which in its turn is connected to the output terminals.
The switching arrangement comprises means for detecting the lamp current and the lamp voltage. The power consumption of the lamp is determined by means of the signals thus detected and is subsequently used as a control signal for controlling the semiconductor switching element of the flyback converter. The lamp is thus power-controlled.
The known switching arrangement has the drawback that information about the current through the primary winding of the transformer is obtained by means of a separate measuring resistor in series with the semiconductor switching element. This leads to large losses. Another way of detecting the current through the primary winding is the use of a current transformer. This has the drawback that a transformer is relatively large and impedes the miniaturization of the switching arrangement.
It is an object of the invention to provide a measure eliminating the drawback described.
According to the invention, the switching arrangement of the type described in the opening paragraph is characterized in that it comprises means M for generating a control signal S for the control circuit in dependence upon the secondary winding voltage and a current through the secondary winding.
The switching arrangement according to the invention has the surprising advantage that a direct current measurement by the primary winding can be dispensed with. In an advantageous embodiment of the switching arrangement, the means M comprise a sense circuit for generating a signal S1 in dependence upon the secondary winding voltage. The means M preferably also comprise a current-sense circuit for detecting a current through the secondary winding. It is then advantageous if the current-sense circuit is connected to an output of the sense circuit by means of a sample and hold switching circuit. It is thus relatively easily possible to correct the signal S1 for the occurrence of a DC voltage across the secondary winding. In an advantageous embodiment, this is realized in that the switching arrangement comprises a switching element which is alternately switched to a conducting and a non-conducting state by means of an inverted switching signal of the control circuit.
An advantageous embodiment of the sense circuit is constituted by a capacitive sense element which is connected to the secondary winding by means of an impedance. A main electrode of the switching element is preferably-connected between the capacitive sense element and the impedance. In this way, the desired control signal is formed by means of a relatively simple construction of the means M. This contributes to the miniaturization of the switching arrangement.
In an advantageous embodiment of the switching arrangement according to the invention, the control circuit for generating a switching signal for controlling the. semiconductor switching element comprises means for maximizing, with respect to time, a switching state of the semiconductor switching element. It is thereby achieved that the time during which the semiconductor switching element is in the conducting or non-conducting state is limited under circumstances of, for example, a very low lamp voltage or a relatively low power supply voltage.
These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter.